What causes "ick" in a fish tank and how can it be prevented or treated effectively?

"Ick" in a fish tank is caused by a parasite called Ichthyophthirius multifiliis. It can be prevented by maintaining good water quality, avoiding stress to the fish, and quarantining new fish. To treat it effectively, you can raise the water temperature slowly to around 86F and use medication specifically designed to treat ich.